1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus in which toner images are formed according to image data using electrophotography.
2. Discussion of the Background
A variety of image forming apparatus such as digital copiers, printers, and facsimile machines have been in practical use. Requests of customers for these image forming apparatus are to have good reliability (i.e., good durability), a low cost, good operation ability, and to produce images having good image qualities.
Among various constitutional devices of such image forming apparatus, developing devices have a great influence on these properties. Among various developing methods using a dry toner for use in image forming methods using electrophotography and the like, two-component developing methods are preferable because of having high-speed developing ability. Therefore, the two-component developing methods are mainly used for image forming apparatus such as copiers and laser printers as a main device.
A typical two-component developing method is as follows:
(1) a two-component developer including a toner and a carrier is supplied to a non-magnetic surface of a developer supporter inside of which a magnet is provided so that the surface of the developer supporter can hold the developer like a brush (hereinafter this brush is referred to as a magnetic brush); PA1 (2) the magnetic brush is brought into contact with or set closely to an image supporter while an electric field is formed between the image supporter and the developer supporter by applying a bias to the developer supporter; and PA1 (3) the toner is selectively adhered to electrostatic latent images formed on the image supporter, resulting in formation of toner images on the image supporter. Thus, developing is performed.
A two-component developer includes a toner and a carrier. The carrier is magnetic so as to be held and fed on the surface of a developer supporter by a magnetic force of a magnet set inside the developer supporter. The toner has a charge due to the friction with the carrier, and is adhered to the carrier surface. The toner on the surface of the carrier, which is fed to the image supporter having electrostatic latent images, is transferred onto the image supporter according to the electrostatic latent images. In general, new toner equal to the amount of the toner exhausted by developing electrostatic latent images is compensated to the developer and mixed with the carrier, which remains in the developer, and thereby the new toner is charged. The compensated toner is then fed together with the carrier and used for developing electrostatic latent images.
At an area (hereinafter referred to as a developing area) at which an image supporter and a developer (i.e., a magnetic brush) supported on a developer supporter contact, a toner is separated from a carrier to develop an electrostatic latent image, and on the other hand a phenomenon which occurs is that charges whose polarity is opposite to that of the toner adhered to the carrier attract the toner adhered to the image supporter by a coulomb force, resulting in re-adhesion of the toner on the image supporter to the carrier. Therefore, a problem which occurs is that a scratched solid image is formed or the rear portion of a rectangular solid image or half tone image is omitted.
In addition, recently a need exists for high density images (i.e., images having a large amount of information), and therefore electrostatic latent images also have a high density. When such high density electrostatic latent images having, for example, a diameter of about 50 .mu.m are developed with a conventional two-component developer, it is difficult to form a toner image having the same dot size, and therefore the resultant toner image has unsatisfactory resolution.
In two-component developing methods, a variety of developing methods, in which a periodic electric field is formed between a developer supporter and an image supporter by applying to the developer supporter a periodically-changing bias (hereinafter referred to as an AC bias) to improve developing ability and image qualities of developed toner images, have been proposed and practically used.
For example, Japanese Laid-Open Patent Publication No. 6-348117 discloses an image forming apparatus in which the conditions of an applied AC bias are changed depending on original images to be reproduced to optimize reproduction of half tone images. In addition, Japanese Laid-Open Patent Publication No. 7-114223 discloses an image forming method in which high quality toner images can be formed by developing digital latent images with a developer, which includes a carrier and a toner and each of which has a small diameter, while applying an AC bias having a high frequency to the developer supporter. It is described in the publication that by using such an image forming method, background fouling, reproducibility of characters, unevenness of solid images, and omission of a rear edge portion of half tone images can be improved and toner images having good image qualities, which are the same as or better than those of photographic images or print images can be obtained.
However, these image forming apparatus and method have the following problem. When an AC bias is applied, a problem which occurs is that white spots are formed in the resultant toner images. The reason is considered to be that discharging occurs locally between the developer supporter and the image supporter due to large potential difference therebetween, and a part of the electrostatic latent images to be developed is discharged, resulting in formation of white spots in the resultant toner images. This will be explained referring to FIG. 8. As can be understood from FIG. 8, the greater the potential difference, V.sub.pu -V.sub.L, between the highest potential V.sub.pu in the potential of the developer supporter and the lowest potential in the potential V.sub.L of the image supporter, the higher the probability of occurrence of white spots in the resultant toner images. In FIG. 8, characters V.sub.D and V.sub.L denote a potential of a non-image area of the image supporter which is not exposed to light and a potential of an image area of the image supporter which is exposed to light, respectively.
In addition, these image forming apparatus and method have another problem in that the rear edge of a solid image is omitted without being developed (hereinafter this problem is referred to as rear edge omission). FIG. 9 is a schematic view illustrating how the rear edge omission occurs. In FIG. 9, an image supporter 14 and a developer supporter 31 move in a direction indicated by an arrow a and a direction shown by an arrow b, respectively. The linear speed of the developer supporter 31 is set to be faster than that of the image supporter 14 to supply a large amount of developer to the image supporter 14, which results in increase of image density of the resultant toner images. Therefore, the magnetic brush (i.e., the developer) on the developer supporter 31 develops electrostatic latent images on the image supporter 14 while overtaking the electrostatic latent images. Under such conditions, when the magnetic brush contacts a non-image area of the image supporter 14 at an upstream side of the developing area, the toner located in the top portion of the magnetic brush is released from the surface of the image supporter 14 by a force in a direction toward the developer supporter 31, which is shown by an arrow c, due to the electric field formed in the developing area. Therefore, the longer the time during which the magnetic brush contacts non-image areas of the image supporter 14, the toner content in a portion of the magnetic brush near the image supporter 14 decreases. When such a magnetic brush moves toward a downstream side of the developing area and contacts an image area of the image supporter 14, toner on the rear edge of the image area, which has been developed with the toner, is electrostatically attracted to the magnetic brush as shown by an arrow d. Therefore, the toner at the rear edge of the image area is omitted. Thus the rear edge omission problem occurs. On the other hand, the toner content of the top portion of the magnetic brush increases again. Even when such a magnetic brush further moves toward a more downstream side, the magnetic brush does not attract the developed toner.
In addition, when an AC bias is applied, a force is at work from the image supporter 14 toward the developer supporter 31 due to the potential difference V.sub.PL -V.sub.D between a minimum potential V.sub.PL of the developer supporter 31 and the highest potential V.sub.D (i.e., the potential of a non-image area) of the image supporter 14. Therefore, the rear edge omission further worsens.
Because of these reasons, a need exists for an image forming apparatus in which high density toner images can be clearly reproduced without producing undesired images such as rear edge omission.